Temperature in an oven can cause many problems in baking even if you follow the recipe to a T.

Lots of ovens like mine are not calibrated properly so sometimes the 100 degrees setting on an oven may not necesarily be 100 degrees but less in most casees.

Not sure how to calibrate it apart from testig it first by stickng a thermometer in and if it's off adjust it until you get waht setting on your oven will be equivalent to the recipe recommendation or call a professional.

this is just one thought on what can go wrong with baking and if temperature is not the problem then something else might be.

Not enough mixing to release the gluten, too much mixing and gluten release.

The one egg used may have been too small.

Best is to try another using the same recipe and see how it comes out then you can start investigating stuff and eliminating them one by one until maybe you may even end up concluding that he recipe proportions are at fault and it is just not a fullproof recipe and when you try another recipe, this may proven when you see better results with the new recipe etc.

Try a different brand baking powder and baking soda etc.

I never remembered banana bread rising to a size double of the inital size though as the mixture is more like a dense cake rather than bread.

banana bread isn't suppose to rise that much
the leavening agent is baking soda, not yeast
it's not logical to have it double in size. if it did, everything would spill out because this is a quick bread and its a batter not a dough
the main reason that i can think of is the amount of banada and nuts that you put in.
because this recipe is not in baker's percentage, it is very unaccurate. how much exactly is 1/2 a cup of nuts? 100g? 150? 75? who knows. the same thing goes for bananas
too much of these two ingredients would weight it down. you should try decreasing these two ingredients
as for gluten, mix as little as possible just to incorporate the flour. Quick breads are suppose to be muffin like, the less gluten development the better.

From my experience, don't over mix/beat the mixture. Beating it too much will make it rise very poorly. But it's never supposed to rise a whole lot to begin with. As soon as it looks like it's mixed, you can stop. Don't go nuts with mixing it

I forgot to mention that proper procedure is very important as most banana breads follow a similar procedure to that when making a cake so you need to cream the butter and sugar first then beat in the eggs and add the banana and then add the dry ingredients which are usually all mixed together.

Mixing by hand is always best to prevent overmixing with an automated mixer which can often lead to a stiffening of the batter with the release of too much gluten and so the bread may not rise as it should as the stiffness from the gluten bond keeps it down possibly

This is one thing to keep in mind:
Some recipies give you the ingredients for 2 loafs not one but it's not obvious.
I once had the same problem and that was because I poured double the amount of ingredients into the bread pan when I baked it.

the one with baking soda and yogurt would rise a bit more because baking soda needs an acid to activate (yougurt it that case) baking powder is baking soda with an acid in it (cream of tarter) butter milk would work instead of yogurt

Jareds_Mommy wrote:the one with baking soda and yogurt would rise a bit more because baking soda needs an acid to activate (yougurt it that case) baking powder is baking soda with an acid in it (cream of tarter) butter milk would work instead of yogurt